Wideband direct DCO modulator with in-band compensation

Abstract

A Direct VCO (DCO) modulation apparatus and method that provides a wideband modulated signal output. The wideband response is obtained via signal processing to counteract a high-pass frequency characteristic as seen from the VCO modulation input. That is, low frequency components of data signals are compensated before being applied to a VCO input. The high-pass characteristic in combination with the compensated signal provides a relatively flat, wideband frequency response of the DCO modulator.

We claim: 
     
       1. An apparatus comprising:
 a phase locked loop (PLL) having a VCO-output high-pass characteristic, the PLL comprising:
 a voltage controlled oscillator (VCO) having a PLL VCO control input and a modulation VCO control input; and, 
 a phase locked loop controller coupled to the PLL VCO control input and configured to output a PLL control signal to the PLL VCO control input; 
 
 a signal phase generator configured to generate a phase signal of a desired information-modulated transmit signal; 
 a compensation filter configured to generate a compensated phase signal by increasing low-frequency components of the phase signal to compensate for the VCO-output high-pass characteristic and the VCO gain characteristic; 
 a phase derivative generator having an input coupled to the compensation filter, the phase derivative generator configured to receive the compensated phase signal and to convert the compensated phase signal to a compensated phase derivative modulation signal, and having a modulation control output coupled to the modulation VCO control input for providing the compensated phase derivative modulation signal to the VCO. 
 
     
     
       2. The apparatus of  claim 1  wherein the VCO comprises a first variable capacitance connected to the PLL VCO control input and a second variable capacitance connected to the modulation VCO control input. 
     
     
       3. The apparatus of  claim 2  wherein the first variable capacitance and the second variable capacitance are selected from the group consisting of varactors and switched capacitor banks. 
     
     
       4. The apparatus of  claim 1  wherein the VCO further comprises a variable capacitance and a combiner configured to combine the PLL control signal and the compensated phase derivative modulation signal and to apply the combined control signal to the variable capacitance. 
     
     
       5. The apparatus of  claim 4  wherein the combiner is an analog voltage summer. 
     
     
       6. The apparatus of  claim 4  wherein the PLL controller further comprises a digital filter, and wherein the PLL control signal is a digital signal and wherein the compensated phase derivative modulation signal is a digital signal, and the combiner is digital summer. 
     
     
       7. The apparatus of  claim 6  wherein the variable capacitance is a switched capacitor bank. 
     
     
       8. The apparatus of  claim 6  further comprising a digital-to-analog converter configured to convert an output of the digital summer to an analog control signal and to apply the analog control signal to the variable capacitance. 
     
     
       9. The apparatus of  claim 1  wherein the PLL controller further comprises a digital filter, and wherein the PLL control signal is a digital signal applied to a first set of switchable capacitors via the PLL VCO control input and wherein the compensated phase derivative modulation signal is a digital signal applied to a second set of switchable capacitors via the modulation VCO control input. 
     
     
       10. The apparatus of  claim 1  wherein the compensation filter substantially cancels the VCO-output high-pass characteristic. 
     
     
       11. The apparatus of  claim 1  wherein the compensation filter is based upon an estimate of the VCO-output high-pass characteristic. 
     
     
       12. The apparatus of  claim 1  wherein the signal phase generator is configured to process an inphase baseband signal and a quadrature baseband signal. 
     
     
       13. The apparatus of  claim 12  wherein the signal phase generator is a rectangular-to-polar converter. 
     
     
       14. The apparatus of  claim 12  wherein the signal phase generator is a CORDIC processor. 
     
     
       15. The apparatus of  claim 1  further comprising:
 a signal envelope generator configured to generate an envelope signal of a desired information-modulated transmit signal; and, 
 an amplifier having a signal input connected to an output of the VCO, and having a power control input configured to receive the envelope signal. 
 
     
     
       16. The apparatus of  claim 15  wherein the envelope signal is digital and is first processed by a digital-to-analog converter before being provided to the amplifier. 
     
     
       17. The apparatus of  claim 15  wherein the envelope signal is digital and is applied to a digital power amplifier having an array of power transistors. 
     
     
       18. A method comprising:
 generating a phase signal of a desired information-modulated transmit signal; 
 compensation filtering the phase signal to generate a compensated phase signal by increasing low-frequency components of the phase signal to compensate for a VCO-output high-pass characteristic; 
 generating a compensated phase derivative modulation signal by converting the compensated phase signal to a compensated phase derivative modulation signal; 
 providing the compensated phase derivative modulation signal to a modulation input of a VCO; 
 generating a PLL VCO control signal using a low pass loop filter operating on a PLL error signal formed using an output of the VCO; 
 applying the PLL VCO control signal to a PLL VCO control input, wherein the PLL VCO control signal operates to provide the VCO-output high-pass characteristic by removing low-frequency variations from an output of the VCO, and wherein the compensated phase derivative modulation signal counteracts the VCO-output high pass characteristic; and, 
 providing the output of the VCO to a signal amplifier. 
 
     
     
       19. The method of  claim 18  wherein providing the compensated phase derivative modulation signal to a modulation input of a VCO comprises providing the compensated phase derivative modulation signal to a variable capacitance. 
     
     
       20. The method of  claim 18  wherein providing the PLL VCO control signal to a PLL VCO control input comprises providing the PLL VCO control signal to a variable capacitance. 
     
     
       21. The method of  claim 18  further comprising generating a combined control signal by combining the PLL control signal and the compensated phase derivative modulation signal and applying the combined control signal to a variable capacitance. 
     
     
       22. The method of  claim 18  further comprising converting at least one of (i) the compensated phase derivative modulation signal, (ii) the PLL VCO control signal or (iii) a combined control signal from a digital signal to an analog control signal and applying the analog control signal to a variable capacitance. 
     
     
       23. The method of  claim 18  wherein generating a phase signal of a desired information-modulated transmit signal is performed by processing an inphase baseband signal and a quadrature baseband signal. 
     
     
       24. The method of  claim 18  further comprising:
 generating an envelope signal of a desired information-modulated transmit signal; and, 
 controlling an output signal amplitude of an amplifier applying the envelope signal to a power control input of the amplifier. 
 
     
     
       25. An apparatus comprising:
 an oscillator having an oscillator output, a first variable capacitance, and a second variable capacitance; 
 a time-to-digital converter (TDC) connected to the oscillator output configured to generate a frequency error signal at an error output; 
 a loop filter connected to the TDC and configured to low-pass filter the frequency error output signal and to generate a loop VCO error signal at a loop filter output, the loop filter output connected to the first variable capacitance to reduce low-frequency carrier deviations at the oscillator output, including data-modulation-induced low-frequency variations; 
 a low-frequency-peaking compensation filter connected to the second variable capacitance, the low-frequency-peaking compensation filter configured to generate a compensated phase signal by increasing low-frequency components of a desired phase signal to compensate for the reduction of data-modulation-induced low-frequency carrier deviations; 
 a phase derivative generator having an input coupled to the low-frequency peaking compensation filter, the phase derivative generator configured to receive the compensated phase signal and to convert the compensated phase signal to a compensated phase derivative modulation signal, and having a modulation control output coupled to the second variable capacitance. 
 
     
     
       26. The apparatus of  claim 25  wherein the low-frequency-peaking compensation filter operates on a digital signal having a first sampling rate, and the loop filter operates on a digital signal having a second sampling rate, and the first sampling rate is at least five times the second sampling rate. 
     
     
       27. The apparatus of  claim 25  wherein the first variable capacitance includes a coarse-tuning variable capacitance and a fine-tuning variable capacitance. 
     
     
       28. The apparatus of  claim 27  wherein the coarse-tuning capacitance is a varactor and the fine-tuning capacitance is a switched capacitor bank. 
     
     
       29. The apparatus of  claim 27  wherein the coarse-tuning capacitance is controlled by a channel selector. 
     
     
       30. The apparatus of  claim 25  wherein the low-frequency-peaking compensation filter incorporates a channel-specific voltage-tuning constant.